1. Field of the Invention
The present invention relates to a display device, and more particularly to a display device which enhances light emission characteristics of phosphor excited by the irradiation of electron beams having high density.
2. Description of the Related Art
With respect to a display device, various types of display devices have been known. Particularly, with respect to a projection type display device or a so-called projector, a cathode ray tube used in such a display device is required to exhibit high luminance and hence, phosphor which constitutes a screen of the display device is required to exhibit the excellent light emission characteristics upon excitation by electron beams at high density. Further, among display devices having a large screen, recently, a field emission display (FED) which constitute one of flat panel type cathode ray tubes has been developed. This FED is also required to include phosphor which exhibits the excellent light emission characteristics upon excitation by electron beams having high density.
The phosphor excited by electron beams of high density is required to satisfy requirements such as (1) the excitation dependency of light emitting efficiency is small, that is, “luminance saturation” is small, (2) lowering of light emitting efficiency due to temperature elevation (so-called temperature quenching) is small, (3) the time-sequential change of light emitting efficiency is small (luminance life time is long) and (4) color tone change of emitted light is small. Further, in the competition with other display devices having a large screen such as a projection type display device using a liquid crystal panel, for example, to further enhance the resolution of displayed images, the suppression of scattering of light in a phosphor film has been studied. In such a study, a method which decreases a film thickness of the phosphor film by using mainly phosphor formed of small particles is adopted and this tendency is steadily increasing recently.
Particularly, many studies have been made with respect to green phosphor which largely contributes to the luminance of images. For example, in “A Study of Phosphors in the yttrium oxide-silicon dioxide phase system” written by P. J. Born, D. S. Robertson, and P. C. Smith (J. Mat. Sci, Lett., 4, pp 497-501 (1985)), a synthesizing method of zinc yttrium silicate activated by terbium (ZnY2Si2O8:TB3+) phosphor and some evaluation results on the method are disclosed.
In the above-mentioned non-patented literature, there is disclosed a technique which mixes oxide raw materials of high purity in a wet form and fills the dried materials in a plasma crucible and synthesizes the materials at a high temperature. Further, in this non-patented literature, evaluation results enumerated in following paragraphs (a) to (f) are described.                (a) When TB is used as an activating agent, it is preferable to perform the synthesizing operation in a nitrogen atmosphere and to set the synthesizing temperature to 135° C.        (b) Green-color phosphor (ZnY2Si2O8:Tb3+) exhibits higher green light emitting efficiency than Y2SiO5:Tb3+ or Y2Si2O7:Tb3+        (c) With respect to the exciting current dependency of the light emission intensity, ZnY2Si2O8:Tb3+ is superior to Y3Al5O12:Tb3+ and exhibits the favorable elongation in the highly excited intensity region.        (d) The deterioration of luminance is not observed under the exciting condition (high current irradiation) which exhibits the luminance of 2000 ft-L.        (e) By setting the mixing ratio of ZnO to 50 mol % or more, it is possible to obtain the light emitting efficiency substantially equal to the light emitting efficiency of ZnY2Si2O8:Tb3+.        (f) Even when the Zn site of ZnY2Si2O8:Tb3+ is substituted by Ca or even when the Y site of ZnY2Si2O8:Tb3+ is substituted by Gd, no noticeable improvement of the light emitting efficiency is observed.        
Further, in Japanese Patent 2,854,635 (concerned with U.S. Pat. Nos. 5,117,401 and 5,363,013), there is a description relating to the luminance lifetime improvement effect obtained by rare earth ions (Eu, Yb, Sm and Tm) of Y3(Al1−xGax)5O12:Tb3+ (here, 0≦x≦1), (Y1−yGdy)2SiO5:Tb3+ (here, 0≦x≦0.45), and InBO3:Tb3+ which constitute Tb3+ activated oxide phosphors.
Among phosphors which constitute the screen of the display device, particularly green-color phosphor (green phosphor) makes the largest contribution to the luminance (brightness) of the screen. In the above-mentioned non-patented literature, although the evaluation of a case in which ZnY2Si2O8 is used as a base material and Tb3+ green phosphor is used as an activating agent is disclosed, the luminance effect orientation is still insufficient. Further, in the above-mentioned patent literature, although the luminance improvement effect due to rare earth ion is described, this luminance improvement effect is also insufficient.
Accordingly, the present invention obtain a display device which can solve the above-mentioned drawbacks of the related art and can further enhance the light emission characteristics of phosphor excited by the irradiation of electron beams of high density.